This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section. Abbreviations that may be found in the specification and/or the drawing figures are defined below at the end of the specification but prior to the claims.
The specification of a communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA) is currently nearing completion within the 3GPP. As specified the DL access technique is OFDMA, and the UL access technique is SC-FDMA.
One specification of interest is 3GPP TS 36.300, V8.12.0 (2010 04), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E UTRA) and Evolved Universal Terrestrial Access Network (E UTRAN); Overall description; Stage 2 (Release 8).” This system may be referred to for convenience as LTE Rel-8 (which also contains 3G HSPA and its improvements). In general, the set of specifications given generally as 3GPP TS 36.xyz (e.g., 36.211, 36.311, 36.312, etc.) may be seen as describing the Release 8 LTE system. Release 9 versions of these specifications have been published, including 3GPP TS 36.300, V9.7.0 (2011-3). Release 10 versions of these specifications have been published, including 3GPP TS 36.300, V10.4.0 (2011-06).
FIG. 1 reproduces FIG. 4-1 of 3GPP TS 36.300 V8.12.0, and shows the overall architecture of the E UTRAN system 2 (Rel-8). The E-UTRAN system 2 includes eNBs 3, providing the E-UTRAN user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE (not shown). The eNBs 3 are interconnected with each other by means of an X2 interface. The eNBs 3 are also connected by means of an S1 interface to an EPC, more specifically to a MME by means of a S1 MME interface and to a S-GW by means of a S1 interface (MME/S-GW 4). The S1 interface supports a many-to-many relationship between MMEs/S-GWs and eNBs.
The eNB hosts the following functions:                functions for RRM: RRC, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both UL and DL (scheduling);        IP header compression and encryption of the user data stream;        selection of a MME at UE attachment;        routing of User Plane data towards the EPC (MME/S-GW);        scheduling and transmission of paging messages (originated from the MME);        scheduling and transmission of broadcast information (originated from the MME or O&M); and        a measurement and measurement reporting configuration for mobility and scheduling.        
Of particular interest herein are the further releases of 3GPP LTE (e.g., LTE Rel-10) targeted towards future IMT A systems, referred to herein for convenience simply as LTE-Advanced (LTE A). Reference in this regard may be made to 3GPP TR 36.913, V8.0.1 (2009 03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Requirements for Further Advancements for E UTRA (LTE-Advanced) (Release 8). A goal of LTE-A is to provide significantly enhanced services by means of higher data rates and lower latency with reduced cost. LTE-A is directed toward extending and optimizing the 3GPP LTE Rel-8 radio access technologies to provide higher data rates at very low cost. LTE-A is part of LTE Rel-10. LTE-A is a more optimized radio system fulfilling the ITU-R requirements for IMT-A while maintaining backward compatibility with LTE Rel-8. Reference is further made to a Release 9 version of 3GPP TR 36.913, V9.0.0 (2009-12). Reference is also made to a Release 10 version of 3GPP TR 36.913, V10.0.0 (2011-06).
As is specified in 3GPP TR 36.913, LTE-A should operate in spectrum allocations of different sizes, including wider spectrum allocations than those of Rel-8 LTE (e.g., up to 100 MHz) to achieve the peak data rate of 100 Mbit/s for high mobility and 1 Gbit/s for low mobility. It has been agreed that carrier aggregation is to be considered for LTE-A in order to support bandwidths larger than 20 MHz. Carrier aggregation, where two or more component carriers (CCs) are aggregated, is considered for LTE-A in order to support transmission bandwidths larger than 20 MHz. The carrier aggregation could be contiguous or non-contiguous. This technique, as a bandwidth extension, can provide significant gains in terms of peak data rate and cell throughput as compared to non-aggregated operation as in LTE Rel 8.
A terminal may simultaneously receive one or multiple component carriers depending on its capabilities. A LTE-A terminal with reception capability beyond 20 MHz can simultaneously receive transmissions on multiple component carriers. A LTE Rel-8 terminal can receive transmissions on a single component carrier only, provided that the structure of the component carrier follows the Rel-8 specifications. Moreover, it is required that LTE-A should be backwards compatible with Rel-8 LTE in the sense that a Rel-8 LTE terminal should be operable in the LTE-A system, and that a LTE-A terminal should be operable in a Rel-8 LTE system.
FIG. 1B shows an example of the carrier aggregation, where M Rel-8 component carriers are combined together to form M×Rel-8 BW (e.g., 5×20 MHz=100 MHz given M=5). Rel-8 terminals receive/transmit on one component carrier, whereas LTE-A terminals may receive/transmit on multiple component carriers simultaneously to achieve higher (wider) bandwidths.
With further regard to carrier aggregation, what is implied is that one eNB can effectively contain more than one cell on more than one CC (frequency carrier), and the eNB can utilize one (as in E-UTRAN Rel-8) or more cells (in an aggregated manner) when assigning resources and scheduling for the UE.
ICIC was introduced in Rel-8/9 of the 3GPP LTE standards. The basic idea of ICIC is keeping the inter-cell interferences under control by RRM methods. ICIC is inherently a multi-cell RRM function that needs to take into account information from multiple cells (e.g., resource usage status and traffic load situation).
Generally, the main target of any ICIC strategy is to determine the resources (e.g., bandwidth and power) available at each cell at any time. Then (and typically), an autonomous scheduler assigns those resources to users. Thus, from the RRC perspective, there are two kinds of decisions: (a) which resources to allocate to each cell, and (b) which resources to allocate to each user. Clearly, the temporality of such decisions is quite different. Whereas user allocations are on the order of milliseconds, the cell allocations span much longer periods of time or can be fixed.
Static ICIC schemes are attractive for operators since the complexity of their deployment is very low and there is no need for new or extra signaling. Static ICIC mostly relies on the fractional reuse concept. This means that users are categorized according to their SINR (essentially according to their inter-cell interference) and different reuse factors are applied to them, being higher at regions with more interference, mostly outer regions of the cells. The total system bandwidth is divided into sub-bands which are used by the scheduler accordingly.
For example, the users may be divided into two categories: Cell Center Users (CCUs) and Cell Edge Users (CEUs). CCUs are the users distributed towards the center of a given cell, whereas CEUs are the users distributed towards the edges of a given cell. CCUs can use all the frequency points to communicate with the base station, while CEUs must use corresponding specified frequency points to ensure orthogonality between different cells (e.g., since CEUs will necessarily be subject to inter-cell interference).
CEUs can be assigned a higher transmission power as the frequency reuse factor is greater than 1. The frequency points are not overlapped at the edges so the adjacent cell interference is small. CCUs frequency reuse factor is 1 as the path loss is small and transmission power is low. Therefore, the interference with adjacent cells is not high either.
Interference avoidance based on frequency-domain partitioning between different cells is of limited benefit for synchronization signals, PBCH, CRSs or control channels (e.g., PDCCH, PCFICH, PHICH). These are needed for initial access to the network and/or thereafter for maintaining the radio link. Therefore, their time-frequency locations are fixed (excepting CRSs, which can use a frequency reuse factor of 3 or 6 depending on the number of antenna ports configured) and frequency partitioning of these channels and signals would not be backwards compatible with Rel-8/9 UEs. However, the interference experienced by the pico cell UEs in a co-channel macro-pico deployment also affects these channels and, if large range expansion is employed, control channel reception at the pico cell UEs may fail, resulting in outage.
Such a need for interference mitigation of the control channels was the motivation for time-domain-based ICIC in Rel-10. The overall objective of eICIC is to mute certain subframes of one layer of cells in order to reduce interference with the other layer. These muted subframes are referred to as ABSs.
ABSs are subframes with reduced DL transmission power and/or activity. Ideally, ABSs would be completely blank in order to remove as much interference as possible. However, one still wants to balance the gains from interference reduction with the loss of transmission resources (e.g., from being unable to transmit PDSCH data in the ABSs). Furthermore, a desire for backwards compatibility means that cells must remain accessible and measurable for Rel-8/9 UEs. CRS is at least transmitted in ABS subframes so legacy UEs can use it for various measurements. In addition, PHICH is also transmitted in ABS subframes to avoid shutting off the corresponding uplink subframes. Nonetheless, even with these transmissions the ABSs can contain much less energy than normal subframes and, thus, reduce interference.
eICIC effectively extends ICIC to DL control in the time domain. eICIC requires synchronization at least between the macro eNB and the low power eNBs in its footprint, such as those eNBs, base stations or access points (e.g., HeNBs) that are serving femto or pico cells, for example. eICIC does not have a negative impact on legacy Rel 8 use (e.g., legacy UEs and legacy users).
RE refers to a UE's ability to connect and stay connected to a cell with low SINR. This is achieved with advanced UE receivers that use DL IC.
Use of both eICIC and RE techniques eliminates coverage holes created by closed HeNBs (e.g., privately operated HeNBs that do not allow for open, public access). Furthermore, these techniques improve load balancing potential for macro networks with low power eNBs and may lead to significant network throughput increase. In addition, these techniques enable more UEs to be served by low power eNBs, which can lead to substantially higher overall network throughput.